Efficient light coupler from off-chip to on-chip waveguides

ABSTRACT

In an embodiment, light from a single mode light source may be deflected into a low index contrast (LIC) waveguide in an opto-electronic integrated circuit (OEIC) (or “opto-electronic chip”) by a 45 degree mirror. The mirror may be formed by polishing an edge of the die at a 45 degree angle and coating the polished edge with a metal layer. Light coupled into the LIC waveguide may then be transferred from the LIC waveguide to a high index contrast (HIC) waveguide by evanescent coupling.

This is a Divisional Application of Ser. No. 12/031,665 filed Feb. 14,2008, which is presently pending which is a continuation application ofand claims priority to Ser. No. 10/421,640, filed Apr. 22, 2003 which ispresently pending.

BACKGROUND

Opto-electronic integrated circuits (OEICs) may incorporate bothelectronic circuits and optical devices, such as integrated waveguides,modulators, switches, and detectors. The optical devices may be usedfor, e.g., optical clock distribution, intra-chip optical signaling, andchip-to-chip communication Both the electronic circuits and opticaldevices may be produced on silicon using complementary metal-oxidesemiconductor (CMOS) fabrication techniques.

Light utilized by optical devices in an OEIC may be introduced into thechip by an external source, such as a vertical cavity surface emittinglaser (VCSEL) or an optical fiber. The light from the external sourcemay have a relatively large mode compared to that of the on-chipwaveguides. The differences in mode size may present difficulties inefficiently coupling the relatively large mode off-chip light source toa small waveguide on the chip.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an opto-electronic chip bonded to aflip chip.

FIG. 2 is a plan view of an optical layer in the opto-electronic chip.

FIG. 3 is a sectional view of an integrated waveguide structure.

FIG. 4 is a plan view of an integrated waveguide structure.

FIG. 5 is a sectional view of adjacent low and high index contrastwaveguides.

FIG. 6 is a chart showing the normalized propagation constant as afunction of the separation of the waveguides of FIG. 5.

FIGS. 7A and 7B are plots showing the localization of the dispersioncurve in the waveguides of FIG. 5.

FIGS. 8A-8J show steps in an exemplary process for fabricating theopto-electronic chip shown in FIG. 1.

FIG. 9 shows an opto-electronic chip according to an alternativeembodiment.

DETAILED DESCRIPTION

FIG. 1 shows an opto-electronic integrated circuit (OEIC) (or“opto-electronic chip”) 100 coupled to a flip chip package 105. The flipchip package may include a light source 110, e.g., a laser or opticalfiber. Modulated light signals from the light source may be deflectedinto a low index contrast (LIC) waveguide 115 by a 45 degree mirror 118.The LIC waveguide may be mode-matched to the light source 110 tominimize coupling loss. Light coupled into the LIC waveguide 110 maythen be transferred from the LIC waveguide to a high index contrast(HIC) waveguide 120 by evanescent coupling.

The HIC waveguide 120 may be laid out in a pattern, e.g., a treestructure, to distribute the light across the chip, as shown in FIG. 2.Photodetectors 125 may convert the light signals into electricalsignals. The electrical signals may be transferred to electroniccircuitry in the chip through electrical interconnects 130 inmetallization layers 135 of the chip 100.

The light source may be a single mode (SM) optical fiber, VCSEL(Vertical Cavity Surface Emitting Laser), or other single modesemiconductor laser. “Mode” refers to the solution of Maxwell's waveequation satisfying the boundary conditions of the waveguide, thusforming a unique pattern of standing wave in the radial direction on thecross section of the waveguide. A mode is characterized by itspropagation constant (eigenvalue of the wave equation). A single modelight source may be appropriate for the relatively small waveguidespresent in the opto-electronic chip.

The light source may be positioned vertically with respect to the deviceside of the chip and placed in close proximity. The light may impinge onthe surface of the chip and be transmitted through a transparentcladding film 150 (e.g., SiO₂) and across the LIC waveguide material115. Anti-reflective (AR) coatings may be provided on the chip surfaceto avoid reflection.

The light may then strike a 45 degree metal mirror and be reflected 90degrees, in the same direction as the waveguide, i.e., parallel to thechip surface. The light may be trapped by total internal reflection andcoupled into the LIC waveguide 115. The index contrast of this waveguide(e.g., the difference between the indexes of refraction of the waveguidecore and the surrounding cladding layer) may be tailored such that themode size is close to that of the fiber to promote efficient coupling,thereby reducing the power requirement for the off-chip light source.

As shown in FIGS. 1 and 2, the LIC waveguide 115 may be larger than theHIC waveguide 120. The mode of the LIC waveguide 115 may more closelymatch the mode of the light source 110. However, the bend radii of HICwaveguides may be much smaller (e.g., less than about 50 microns)compared to LIC waveguides, which may be only able to bend at about 1 mmradius. Having a smaller alloable bend radius allows for more efficientdistribution of light about the chip. Accordingly, the LIC waveguide 115may be used to couple light into the chip, and the HIC waveguide(s) 120may be used for distribution and signaling.

A cross section and a top view of an integrated waveguide are shown inFIGS. 3 and 4, respectively. The waveguide may be an optically guidingcore 305 of a material with refractive index n_(w) surrounded by acladding material with a different index of refraction, n_(c). The highcontrast of the refractive index between the two materials confines alightwave to the waveguide 305. The cladding material may be, e.g.,silicon oxide (SiO₂) (n_(c)≈1.5). The waveguide material may be selectedfrom, e.g., silicon nitride (Si₃N₄) (n_(w)≈2), silicon (Si) (n_(w)≈3),and silicon oxynitride (SiON) (n_(w)≈1.55). Silicon oxynitride may offerdesign flexibility because its refractive index may be varied bychanging the content of nitrogen. The difference in the indexes ofrefraction between the core and the cladding determines the contrast,e.g., high index contrast or low index contrast.

Light may be transferred from the LIC waveguide 115 to the HIC waveguide120 by evanescent coupling. Since the index of the HIC waveguide 120 ishigher than that of the LIC waveguide 115, the light gets coupledthrough the evanescent tail of the low index contrast mode. Alithographically patterned taper 200 may be used at the end of the LICwaveguide to make the transfer occur over a shorter length, as shown inFIG. 2. The interaction length may be designed such that substantiallyall of the light is transferred to the lower HIC waveguide 120.

FIG. 5 shows two single mode waveguides 505, 510 with n_(w1)=1.6 (LIC)and n_(w2)=2.0 (HIC), respectively, and cladding index n_(c)=1.5. Thedistance “d” may be varied from 0.0 to 1.2 microns. FIGS. 6 and 7A-Billustrate modeling simulations for this waveguide configuration. FIG. 6is a chart illustrating the normalized propagation constant as afunction of the separation of waveguides. Each waveguide contains adoubly degenerate effective index when isolated. The upper branch ofdispersion may be asymmetric mode localized to the HIC waveguide 510(modes 0 and 1) and the lower branch corresponds to LIC waveguide (modes2 and 3). FIG. 7A shows that the upper branch of the dispersion curve islocalized in the HIC waveguide and is weakly coupled to the LICwaveguide. FIG. 7B shows that the lower branch of the dispersion curveis localized in LIC waveguide and strongly coupled to HIC waveguide. Thecoupling efficiency ranges from 70% at d=0.0 to 20% at d=0.5 based onthe ratio of peak amplitudes in waveguides.

FIGS. 8A-J show stages in the fabrication of the optical layers and 45degree mirror in the chip according to an embodiment. A lower claddingfilm 800, such as SiO₂ may be deposited on the top of the lastmetallization layer 805 in the chip, which may include electricalinterconnect lines to electronic circuitry in the chip. A core material810 for the HIC waveguide 120, such as Si₃N₄, may be deposited on thelower cladding film 800. The silicon nitride layer may then be etched toform a HIC waveguide pattern. An intermediate cladding layer 815, e.g.,silicon dioxide, may be deposited over the HIC waveguide layer 810.Next, a core material 820 for the LIC waveguide 115 may be deposited onthe intermediate cladding layer 815. The silicon oxynitride layer maythen be etched to form a LIC waveguide pattern. An upper cladding layer825 may then be deposited on the LIC waveguide pattern.

The wafer may then be diced, producing an edge 830. The edge 830 of thedie may be polished to a 45 degree angle edge 835. A thin layer 840 of ametal material such as Al may be applied, e.g., by sputtering orevaporation. Anti-reflective coatings may also be added to the topsurface to reduce reflection. The light source, e.g., an optical fiber,may then be joined to the top of the upper cladding layer 825 of the LICwaveguide, e.g., melting the fiber to adhere to the surface or by use ofan adhesive.

In another embodiment, light may enter the backside surface of the chipand hit a mirror 905 which is polished at 45 degrees, as in FIG. 9. Thisstructure may be used with light having a wavelength greater than about1.2 microns.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the invention. Accordingly, other embodimentsare within the scope of the following claims.

1. A method comprising: fabricating an integrated optical waveguide on asubstrate; polishing an edge of the integrated optical waveguide at anangle; and coating the polished edge of the integrated optical waveguidewith a reflective material to form a mirror positioned to couple lightunto the integrated optical waveguide.
 2. The method of claim 1, whereinpolishing the edge of the integrated optical waveguide comprisespolishing an edge of a semiconductor die.
 3. The method of claim 1,wherein fabricating the integrated optical waveguide comprisesfabricating the integrated optical waveguide substantially parallel to asurface of the substrate.
 4. The method of claim 1, further comprisingcoupling the substrate to a flip chip package so that light of a lightsource is deflected into the integrated optical waveguide.
 5. The methodof claim 10, wherein: fabricating the integrated optical waveguidecomprises fabricating a low index contrast waveguide; and the methodfurther comprises fabricating a high index contrast waveguide adjacentto the low index contrast waveguide, wherein the high index contrastwaveguide is separated from the low index contrast waveguide by adistance that allows light to evanescently couple from the low indexcontrast waveguide into the high index contrast waveguide.
 6. The methodof claim 5, further comprising patterning the high index contrastwaveguide into a distribution pattern.